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Full text of "Text Book Of Mechanical Engineering"

in6                        Appendix VI.

current taken from the brush terminals through a shunt wire
round the magnet, termed shunt-wound: and Q, where two
windings are laid round the magnet, one from the circuit, and one
a shunt, making a compound-wound dynamo.

In the ease of Alternators it has become the fashion to call the
moving part the Rotor, and the standing part the Stator; so
that either armature or magnets may b%revolved under this
nomenclature.

Transformers.   A direct current cannot be easily trans-

E
formed, that is, have the relation g .altered in value while E x C

remains  approximately constant,  and E a 1    The alternating

\^f

current can be easily so treated. Shewn simply in Fig. 982, we
have the principles of the induction coil, where on the primary
side a current of low E and high C is transformed into a high E
and low C in the secondary. The reverse operation is also
possible, so that there are both step-up and step-down trans-
formers.

(!                             Rotary Converters are a means of changing an alternating

current into a direct current, and vice versa, consisting of a single
l'                      dynamo, Fig. 983, having a split commutator at one end of the

armature for D. C, and slip rings at the other end for A* C.    Only
one definite relation is possible between the two sides, as /or
example from 400 or 500 volts D. C. to 300 volts A. C. in a three-
I           ,            phase machine.    If further alteration be required, a static trans-

former T must be put in the alternating circuit. This machine is
remarkably adaptable, however: it may be used (i) to change
existing current to or from either form, (2) to change part of the
current and deliver the rest mechanically to the machine as
motor, (3) as a generator supplying either form of current, or
both.

^ Motor Generators consist of at least one pair of machines,
;                    - one of which is a generator and the other a motor.    In Fig. 984

f,                     the motor M receives an alternating current, and drives the

r     '               dynamo D, from which a direct current is evolved.   Any desired

I                                                       *                                              TT                                                                                *

j                       alteration in the ratio -^ can be procured when first designing.

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